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water-bath.    In a few minutes the yellow crystalline phenyl"
glucosazone (in. p. 204—205°) is deposited.
5. Mix a few drops of a glucose solution with a few drops o^
an alcoholic solution of a-naphthol and pour slowly down the sid^
of the test-tube a few drops of cone, sulphuric acid. A violet
colouration is produced. (Molisch's reaction.) See Appendix*)
p. 271.
Pure Commercial Benzene, obtained from coal-tai"
naphtha, should distil within one degree (80—Sic), and solidify
completely when cooled to o°. Other tests are as follows *
shaken with concentrated sulphuric acid for a few minutes, the
acid should not darken, and a drop of bromine wafer should
not be immediately decolourised. A single distillation over a
few small pieces of sodium, which absorb any traces of water, is
usually a sufficient purification. If the benzene impart a brown
or black colour to the sulphuric acid, it must be repeatedly
shaken with about 20 per cent, of the acid until the latter
becomes only slightly yellow on standing. This is done in a
stoppered separating funnel, and after shaking fora few minutes
the mixture is allowed to settle, and the lower layer of acid
drawn off. The benzene is then shaken two or three times with
water to free it from acid, carefully separated from the aqueous
layer,, and left in contact with fused calcium chloride until the
liquid becomes clear. It is then decanted, frozen in ice, and
any liquid (carbon bisulphide, paraffins) carefully drained off,
and the benzene finally distilled over sodium.
Properties.—Mobile, colourless liquid ; m. p. 5*4° ; b. p. §P'4C ;
sp. gr. 0*874 at 2°°' Coal-tar benzene usually contains a little
thiophene, C4H4S, which may be detected by dissolving a fe\v
crystals of isatin (see p. 229) in concentrated sulphuric acid and
shaking up with the" benzene. If thiophene is present, a blue
colour is produced (indophenin reaction).
Fractional Distillation.—It is often possible to separate
almost completely by a single distillation, two liquids occurring"
together in a mixture when their boiling points lie widely apart.. _
The more  volatile  liquid first passes  over,  the temperature
suddenly rises, and the higher boiling liquid distils.
It is otherwise when a liquid consists of a mixture of sub-
Stances boiling at lifnperatures not very far removed from one